Stainless steel used for oil country tubular goods

ABSTRACT

A stainless steel for an oil country tubular good according to the invention includes, in percent by mass, 0.001% to 0.05% C, 0.05% to 1% Si, at most 2% Mn, at most 0.03% P, less than 0.002% S, 16% to 18% Cr, 3.5% to 7% Ni, more than 2% and at most 4% Mo, 1.5% to 4% Cu, 0.001% to 0.3% rare earth metal, 0.001% to 0.1% sol. Al, 0.0001% to 0.01% Ca, at most 0.05% O, and at most 0.05% N, and the balance consists of Fe and impurities. The stainless steel according to the invention includes REM and therefore has high SCC resistance in a high temperature chloride aqueous solution environment.

TECHNICAL FIELD

The present invention relates to stainless steel and more specifically to stainless steel used for oil country tubular goods for use in gas wells or oil wells.

BACKGROUND ART

Oil or natural gas produced from oil wells or gas wells contains associated corroding gas as such as carbon dioxide gas and hydrogen sulfide. Therefore, oil country tubular goods used for producing oil or natural gas need high corrosion resistance.

Carbon steel or low alloy steel has been used as a steel for oil country tubular goods. As the goods have come to be used in a tougher corroding environment in an oil well or a gas well, SUS420 martensitic stainless steel (13% Cr-based steel) having a Cr content of about 13% or stainless steel having high corrosion resistance such as improved 13% Cr-based steel produced by adding Ni to the 13% Cr-based steel has been used.

Recently, deep oil or gas well drilling has created a demand for oil country tubular goods having higher strength for such deep oil or gas wells. Furthermore, in a deep oil or gas well, a high temperature chloride aqueous solution environment as high as 150° C. or more including hydrogen sulfide and carbon dioxide gas is created, and even higher corrosion resistance than the conventional oil country tubular good is required. In such a high temperature chloride aqueous solution environment including hydrogen sulfide and carbon dioxide gas, two-phase stainless steel having corrosion resistance and strength higher than conventional stainless steel may be used. The two-phase stainless steel however contains a large amount of alloy elements and therefore the manufacturing cost is high.

JP 2005-336595 A (hereinafter referred to as “Patent Document 1”), JP 2006-16637 A (hereinafter referred to as “Patent Document 2”), and JP 2007-332442 A (hereinafter referred to as “Patent Document 3”) propose the use of stainless steel pipes containing less alloy elements than the two-phase stainless steel and having high strength and high corrosion resistance in a high temperature chloride aqueous solution environment including carbon dioxide gas. The stainless steel pipes disclosed by these patent documents each have a greater Cr content than the conventional 13% Cr-based steel, such that the corrosion resistance can be improved.

More specifically, in the disclosure of Patent Document 1, the Cr content of the stainless steel pipe is from 15.5% to 18% which is greater than that of the conventional 13% Cr-based steel. Furthermore, when Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≧11.5 is established, the steel has a two-phase structure including a ferrite phase and a martensite phase, so that the hot workability of the oil country tubular good is improved. The two-phase structure could lower the corrosion resistance, while when Ni, Mo, and Cu that improve the corrosion resistance are added such that Cr+0.65Ni+0.6Mo+0.55Cu−20C≧19.5 is established, the reduction in the corrosion resistance of the oil country tubular good is prevented.

Similarly, according to the disclosure of Patent Document 2, the Cr content of the stainless steel is from 15.5% to 18% and Ni that improves the corrosion resistance is contained. The chemical composition of the stainless steel disclosed by the document is similar to that in Patent Document 1, but Mo is not an essential element, and therefore a less costly alloy design is proposed. In addition, Cu is also an optional element.

The stainless steel disclosed by Patent Document 3 contains 14% to 18% Cr as well as Ni, Mo, and Cu, so that high corrosion resistance is obtained. Furthermore, the steel includes a martensite phase and 3% to 15% austenite phase by volume, and therefore the toughness improves.

The kinds of stainless steel disclosed by Patent Documents 1 to 3 surely contain a larger amount of Cr than the conventional 13% Cr-based steel and alloy elements such as Ni, Mo, and Cu are added, so that the corrosion rate in a high temperature corroding environment is reduced. For example, in an embodiment in Patent Document 1, using a 20 wt % NaCl aqueous solution at 230° C. in a 100 atm CO₂ atmosphere, the corrosion rate (mm/yr) was examined and it was established that the corrosion rate was reduced (see Table 2 in Patent Document 1).

However, it was found based on the inventors' investigation that the use of stainless steel having a high Cr content in a high temperature chloride aqueous solution environment containing carbon dioxide gas lowers the corrosion rate but SCC (Stress Corrosion Cracking) is more likely to be caused.

In conventional stainless steel such as 13% Cr steel, the corrosion rate is extremely high in a high temperature chloride aqueous solution environment. Therefore, while general corrosion is generated, SCC that is local cracking is not generated. On the other hand, when the Cr content is larger than that in the conventional stainless steel, the corrosion rate is lowered as disclosed by Patent Documents 1 to 3. The reduction in the corrosion rate is caused by a passive film that forms on the surface of the stainless steel. However, the passive film is locally weakened and destroyed in a high temperature environment. The destroyed part is more likely to dissolve, and this dissolution is probably the cause for SCC.

Therefore, in stainless steel used in a high temperature chloride aqueous solution environment containing carbon dioxide gas, it is necessary not only to reduce the corrosion rate but also to improve the SCC resistance.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide stainless steel for an oil country tubular goods having high corrosion resistance in a carbon dioxide gas contained, high temperature chloride aqueous solution environment at 150° C. or higher. More specifically, it is to provide stainless steel for an oil country tubular goods having a reduced corrosion rate and high SCC resistance in a carbon dioxide gas contained, high temperature chloride aqueous solution environment.

The inventors considered that it would be necessary to add at least 16 mass % Cr and a small amount of Mo to steel in order to reduce the corrosion rate in a carbon dioxide gas contained, high temperature chloride aqueous solution environment at 150° C. or higher. However, Cr and Mo are ferrite forming elements and therefore if at least 16 mass % Cr and a small amount of Mo are contained, a major part of the structure of the steel becomes a ferrite phase and therefore high strength cannot be obtained.

On the other hand, an austenite phase at high temperatures is stabilized by adding Ni that is an austenite forming element, so that a martensite phase is formed by quenching and a high strength steel structure is obtained. However, if the amount of Ni is too large, the starting temperature for martensite transformation (Ms point) is lowered and therefore martensite transformation is not generated even at room temperatures, so that high strength is not provided. Therefore, when the Ni content is appropriately adjusted, a structure mainly including a martensite phase and about at least 10% ferrite phase by volume is formed and high strength can be provided.

The copper (Cu) effectively enhances a ferrite phase, and therefore a high strength structure can be provided by adding Cu. In addition, Cu reduces the corrosion rate in a high temperature chloride aqueous solution environment and improves the SCC resistance.

Based on the foregoing findings, the inventors concluded that stainless steel having prescribed strength and reduced corrosion rate can be provided when the steel contains 16% to 18% Cr, more than 2% and not more than 4% Mo, 3.5% to 7% Ni, and 1.5% to 4% Cu.

The inventors also found that by adding at least a prescribed amount of an earth rare metal (REM) in the chemical composition described above, high SCC resistance results even in a carbon dioxide gas contained, high temperature chloride aqueous solution environment. Now, this will be described in detail.

The inventors prepared stainless steel having the chemical compositions in Table 1 and these kinds of stainless steel were evaluated for their SCC resistance.

TABLE 1 Chemical composition (unit: mass %, the balance consisting of Fe and impurities) Steel No. C Si Mn P S Cu Cr Ni Mo Al 0.019 0.31 0.51 0.016 0.0009 1.9 17.1 3.9 2.4 A2 0.018 0.30 0.55 0.015 0.0010 2.0 17.2 4.2 2.5 A3 0.021 0.29 0.52 0.017 0.0010 2.1 16.9 4.1 2.5 A4 0.020 0.29 0.49 0.016 0.0008 2.0 17.0 4.0 2.6 A5 0.019 0.31 0.51 0.015 0.0010 1.9 17.2 4.1 2.4 A6 0.019 0.31 0.50 0.016 0.0009 1.9 17.1 4.1 2.4 Steel No. sol.Al Ca N O REM Al 0.029 0.0010 0.020 0.003 0.0001 A2 0.030 0.0008 0.018 0.005 0.0002 A3 0.029 0.0013 0.016 0.006 0.0005 A4 0.028 0.0016 0.019 0.003 0.0011 A5 0.032 0.0011 0.022 0.005 0.0028 A6 0.029 0.0009 0.018 0.003 0.03

Referring to Table 1, in the stainless steel grades with Nos. A1 to A6, the chemical compositions are the same except for REM. The REM content is different among the numbered stainless steel grades in the range from 0.0001% to 0.03%. Furthermore, these numbered stainless steel grades were subjected to quenching-tempering such that the yield stress of each kind of stainless steel was adjusted in the range from 860 MPa to 900 MPa. The structures of these numbered stainless steel grades include, in volume percentage, 60% martensite phase, 30% ferrite phase, and 10% austenite phase.

A specimen for four-point bending test having a length of 75 mm, a width of 10 mm, and a thickness of 2 mm was sampled from each of the numbered stainless steel grades. The sampled specimens were subjected to a bending load by four-point bending. At the time, the bending amount of each specimen was determined according to ASTM G39 such that stress applied on each specimen was equal to the yield stress of each specimen.

Each bent specimen was immersed for one month in a 25 wt % NaCl aqueous solution in an autoclave at 204° C. (400 F) having CO₂ enclosed therein under a pressure of 30 atm. After the immersion for one month, each specimen was examined for the presence of SCC. More specifically, a longitudinal section of each specimen was observed with a 100× magnification optical microscope and determined for the presence/absence of SCC by visual inspection.

The test result is given in FIG. 1. In FIG. 1, the abscissa represents the REM content (% by mass) and the ordinate represents the presence/absence of SCC. In FIG. 1, “” in the “SCC present” on the ordinate indicates the presence of SCC, while “” in the “no SCC” indicates the absence of SCC. As can be clearly seen from FIG. 1, when the REM content was not less than 0.001%, no SCC was generated even in a carbon dioxide contained, high temperature chloride aqueous solution environment. While how REM improves the SCC resistance is not clearly known, this may be for the following reason.

As a result of microscopic observation of stainless steel specimens that had SCC in the above-described tests, it was found that SCC was originated from a pit and propagated along an prior austenite grain boundary in a martensite predominant structure. This may suggest that the accumulation behaviors of dislocations toward the prior austenite boundary under stress and crack propagation are somehow correlated. Then, REM probably has some effect on the accumulation behaviors of dislocations toward the prior austenite boundary and the SCC resistance of the stainless steel containing at least 0.001% REM may be improved. Note that the stainless steel grades with Nos. A1 to A3 contained 0.0008% to 0.0013% Ca but their REM contents were less than 0.001% and therefore SCC was generated. Therefore, at least 0.001% REM content contributed to the improvement of the SCC resistance more than Ca did.

The inventors have completed the following invention based on the foregoing findings.

Stainless steel used for an oil country tubular goods according to the invention includes, in percent by mass, 0.001% to 0.05% C, 0.05% to 1% Si, at most 2% Mn, at most 0.03% P, less than 0.002% S, 16% to 18% Cr, 3.5% to 7% Ni, more than 2% and at most 4% Mo, 1.5% to 4% Cu, 0.001% to 0.3% rare earth metal, 0.001% to 0.1% sol. Al, 0.0001% to 0.01% Ca, at most 0.05% O, and at most 0.05% N, and the balance consists of Fe and impurities.

The stainless steel according to the invention preferably further includes, in place of a part of Fe, at least one selected from the group consisting of at most 0.5% Ti, at most 0.5% Zr, at most 0.5% Hf, at most 0.5% V, and at most 0.5% Nb.

In this way, the generation of a pit attributable from a Cr depleted layer can be reduced.

The stainless steel described above preferably has a structure including, in volume percentage, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.

The stainless steel according to the invention preferably has a yield stress of at least 654 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the contents of rare earth metals in stainless steel and SCC.

BEST MODE TO CARRY OUT THE INVENTION

Now, embodiments of the invention will be described in detail. Stainless steel according to the invention is applicable to oil country tuber goods for use in a carbon dioxide gas contained, high temperature chloride aqueous solution environment at 150° C. or more. Hereinafter, this carbon dioxide gas contained, high temperature chloride aqueous solution environment at 150° C. or more will be simply referred to as “high temperature chloride aqueous solution environment.”

1. Chemical Composition

The stainless steel according to the invention has the following chemical composition. Hereinafter, “%” related to elements means “% by mass.”

C, 0.001% to 0.05%

Carbon (C) forms carbide with Cr and lowers the corrosion resistance of steel in a high temperature chloride aqueous solution environment. Therefore, the C content is preferably as small as possible. Therefore, the upper limit for the C content is 0.05%. Note that the lower limit for the C content that can substantially be controlled is 0.001%.

Si: 0.05% to 1%.

Silicon (Si) deoxidizes steel in refining process. To obtain the effect, the lower limit for the Si content is 0.05%. On the other hand, an excessive Si content not only saturates the deoxidizing effect but also lowers the hot workability of the steel. Therefore, the upper limit for the Si content is 1%.

Mn: 2% or Less

Manganese (Mn) improves the strength of the steel. However, an excessive Mn content is more likely to cause segregation in the steel. The segregation in the steel lowers the toughness of the steel and also lowers the SCC resistance in a high temperature chloride aqueous solution environment. Therefore, the Mn content is not more than 2%. The Mn content is preferably not less than 0.2% in order to improve the strength. However, if the Mn content is less than 0.2%, the strength of the steel is improved to some extent.

P: 0.03% or Less

Phosphorus (P) is an impurity and lowers the SSC (sulfide stress cracking) resistance and the SCC resistance in a high temperature chloride aqueous solution environment. Therefore, the P content is preferably as small as possible. The P content is therefore not more than 0.03%.

S: Less than 0.002%

Sulfur (S) combines with Mn or the like and forms an inclusion. The formed inclusion becomes an origin for a pit or SCC and lowers the corrosion resistance of the steel. In addition, S lowers the hot workability of the steel. Therefore, the S content is preferably as small as possible. Therefore, the S content is less than 0.002%.

Cr: 16% to 18%

Chromium (Cr) is an essential element that improves the corrosion resistance in a high temperature chloride aqueous solution environment. In order to achieve high SCC resistance in the high temperature chloride aqueous solution environment, the lower limit for the Cr content is 16%. On the other hand, since Cr is a ferrite forming element, an excessive Cr content increases the ratio of a ferrite phase in the steel structure and lowers the strength of the steel. Furthermore, it lowers the ratio of a residual austenite phase, which lowers the toughness of the steel. Therefore, the upper limit for the Cr content is 18%. The Cr content is preferably from 16.5% to 17.5%.

Ni: 3.5% to 7%

Nickel (Ni) improves the corrosion resistance in a high temperature chloride aqueous solution environment and also improves the toughness of the steel. In order to obtain these effects, the lower limit for the Ni content is 3.5%. On the other hand, Ni is an austenite forming element and an excessive Ni content increases the ratio of a residual austenite phase in the structure of the steel, which lowers the strength of the steel. Therefore, the upper limit for the Ni content is 7%. The Ni content is preferably from 3.5% to 6.5%, more preferably from 3.8% to 5.8%.

Cu: 1.5% to 4%

Copper (Cu) lowers the dissolution rate of the steel in a high temperature chloride aqueous solution environment and also improves the SCC resistance of the steel. In addition, Cu strengthens a ferrite phase in the structure of the steel. In order to obtain these effects, the lower limit for the Cu content is 1.5%. On the other hand, an excessive Cu content lowers the hot workability of the steel. Therefore, the upper limit for the Cu content is 4%. The Cu content is preferably from 1.5% to 3.0%, more preferably from 1.5% to 2.5%.

Mo: More than 2% and not More than 4%

Molybdenum (Mo) improves the pitting corrosion resistance and the SCC resistance of the steel when it coexists with Cr. In order to obtain the effects, the Mo content is more than 2%. On the other hand, Mo is a ferrite forming element and therefore an excessive Mo content increases the ratio of a ferrite phase in the structure of the steel, which lowers the strength. Therefore, the Mo content is not more than 4%. The Mo content is preferably from 2.1% to 3.3%, more preferably from 2.3% to 3.0%.

Sol. Al: 0.001% to 0.1%

Aluminum (Al) deoxidizes steel in refining process. In order to obtain the effect, the lower limit for the Al content is 0.001%. On the other hand, an excessive Al content causes a large amount of an alumina inclusion to be generated in the steel, which lowers the toughness of the steel. Therefore, the upper limit for the Al content is 0.1%. Note that the Al content in the specification means the content of acid soluble aluminum (sol.

Ca: 0.0001% to 0.01%

Calcium (Ca) deoxidizes steel in refining process. In addition, Ca improves the hot workability. In order to obtain these effects, the lower limit for the Ca content is 0.0001%. On the other hand, an excessive Ca content causes a large amount of an inclusion such as CaO to be generated in the steel, which lowers the toughness of the steel. Furthermore, the inclusion such as CaO forms an origin for a pit. Therefore, the upper limit for the Ca content is 0.01%.

N: 0.05% or Less

Nitrogen (N) stabilizes an austenite phase and also improves the pitting corrosion resistance. On the other hand, an excessive N content causes various nitrides to be formed in the steel, which lowers the toughness of the steel. Therefore, the N content is not more than 0.05%. In order to effectively obtain the effect, the lower limit for the N content is preferably 0.005%.

O: 0.05% or Less

Oxygen (O) is an impurity and combines with another element to form oxide, which lowers the toughness and the corrosion resistance of the steel. Therefore, the O content is preferably as small as possible. Therefore, the O content is not more than 0.05%.

Rare Earth Metals: 0.001% to 0.3%

Rare earth metals (REM) are important elements according to the invention. The REM improve the SCC resistance in a high temperature chloride aqueous solution environment as described above. In order to obtain the effect, the lower limit for the REM content is 0.001%. On the other hand, an excessive REM content saturates the effect. Therefore, the upper limit for the REM content is 0.3%. The REM content is preferably from 0.001% to 0.1%, more preferably from 0.001% to 0.01%.

Note that the REM according to the invention refer to yttrium (Y) with atomic number 39 and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

The stainless steel according to the invention contains at least one of the above REM. Therefore, the REM content means the total content of at least one selected from the plurality of REM described above.

The balance of the chemical composition includes Fe and impurities.

The stainless steel according to the invention contains at least one selected from the group consisting of Ti, Zr, Hf, V, and Nb in place of a part of Fe if necessary.

Ti: 0.5% or less

Zr: 0.5% or less

Hf: 0.5% or less

V: 0.5% or less

Nb: 0.5% or less

Titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), and niobium (Nb) are not essential elements and added as optional elements. These elements each fix C and reduce the generation of Cr carbide. Therefore, the generation of a pit attributable to a Cr depleted layer formed around Cr carbide is reduced and the SCC sensitivity is reduced. However, an excessive content of any of these elements lowers the toughness of the steel. Therefore, the upper limits for the contents of these elements are each 0.5%. In order to effectively obtain the above-described effect, the lower limits for the contents of these elements are each preferably 0.005%. Note however that if the contents of these elements are less than the preferable lower limit, the above-described effect is obtained to some extent.

2. Manufacturing Method

The stainless steel according to the invention can have the following structure by carrying out quenching-tempering as heat treatment, so that the corrosion resistance as intended and strength necessary when it is used as oil country tubular goods can be provided. Now, a method of manufacturing a stainless steel pipe according to the invention will be described by way of example.

Steel having the above-described chemical composition is melted and made into a billet. The produced billet is subjected to hot working and made into a stainless steel pipe. A Mannesmann method for example is employed as the hot working to make a seamless steel pipe. Note that the hot working may be hot extruding or hot forging.

The produced stainless steel pipe is subjected to quenching and tempering. At the time, the preferable quenching temperature is from 900° C. to 1200° C., and the preferable tempering temperature is from 450° C. to 650° C.

3. Structure

The structure of the stainless steel produced by the above-described method includes, in percent by volume, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.

Now, the volume percentage of the ferrite phase is obtained by the following method. A specimen having its surface polished is etched using a mixture solution of aqua regia and glycerin. Using the etched specimen, the area ratio of the ferrite phase at the specimen surface is measured by a point-counting method according to JISG0555. The measured area ratio is used as a volume percentage. The volume percentage of the residual austenite phase is measured by X-ray diffraction.

Note that in the structure of the stainless steel, the portion other than the ferrite phase and the residual austenite phase is mainly a tempered martensite phase. Carbide, nitride, boride and a Cu phase may be included other than the martensite phase.

The stainless steel according to the invention has the above-described structure, so that the yield stress is not less than 654 MPa (that corresponds to 95 ksi). The yield stress may be adjusted to 758 MPa (that corresponds to 110 ksi) or more and further to 862 MPa (that corresponds to 125 ksi) or more. The yield stress in this specification refers to 0.2% offset yield stress based on the ASTM standard.

The stainless steel according to the invention has high toughness since it contains the residual austenite phase as much as the above-described volume percentage in the structure.

EXAMPLES

A plurality of kinds of stainless steel having various chemical compositions were produced and examined for their SCC resistance in a high temperature chloride aqueous solution environment.

Manufacture of Specimens

A plurality of kinds of stainless steel having the chemical compositions in Table 2 were melted.

TABLE 2 Chemical composition (unit: mass %, the balance consisting of Fe and impurities) Steel No. C Si Mn P S Cu Cr Ni Mo sol.Al 1 0.022 0.36 0.51 0.017 0.001 2.4 16.3 3.6 2.4 0.014 2 0.011 0.39 0.65 0.018 0.0008 1.6 17.6 6.3 2.1 0.039 3 0.018 0.38 1.28 0.019 0.0009 3.3 17.2 3.9 2.2 0.029 4 0.032 0.84 1.79 0.017 0.001 1.7 16.3 4.5 3.5 0.021 5 0.042 0.29 0.23 0.011 0.0015 3.5 17.8 6.2 3.6 0.033 6 0.039 0.11 0.59 0.019 0.0018 1.8 16.2 4.2 2.3 0.018 7 0.022 0.36 0.55 0.022 0.0009 2.3 16.4 4.5 2.6 0.015 8 0.025 0.52 0.68 0.028 0.0008 1.6 16.5 5.6 2.5 0.029 9 0.019 0.48 0.66 0.019 0.0009 1.9 17.2 6.1 2.5 0.036 10 0.018 0.33 0.55 0.017 0.001 2.1 16.6 5.9 2.8 0.055 11 0.023 0.22 0.49 0.015 0.001 2.8 17.5 4.8 2.7 0.057 12 0.026 0.28 0.48 0.011 0.0009 2.4 16.8 4.2 3.2 0.022 13 0.021 0.35 0.44 0.018 0.0008 2.3 16.9 3.7 1.9 0.028 14 0.018 0.39 0.41 0.017 0.0008 2.3 15.1 3.8 2.3 0.019 15 0.023 0.32 0.49 0.015 0.001 1.3 17.2 4.5 2.5 0.033 16 0.025 0.33 0.51 0.014 0.001 2.3 17.5 4.4 2.3 0.036 17 0.021 0.35 0.55 0.014 0.0009 2.2 16.9 3.1 2.8 0.039 Steel No. Ca N O REM Ti Zr Hf V Nb 1 0.0003 0.019 0.005  0.03 a) — — — — — 2 0.0005 0.044 0.018 0.002 a) — — — — — 3 0.001 0.011 0.003 0.001 a) — — — — — 4 0.0015 0.016 0.004 0.008 c) — — — — — 5 0.0005 0.008 0.004 0.019 b) — — — — — 6 0.0008 0.015 0.005  0.22 b) — — — — — 7 0.0012 0.026 0.004 0.015 c) — — — 0.05 — 8 0.0029 0.022 0.008 0.011 c) 0.04 — — — — 9 0.0005 0.018 0.005 0.007 a) — 0.02 — 0.09 — 10 0.0002 0.019 0.003 0.002 a) — — 0.01 0.08 — 11 0.0009 0.017 0.004 0.009 c) — — — 0.11 0.08 12 0.001 0.021 0.006 0.026 b) 0.15 — — 0.04 0.13 13 0.0012 0.019 0.005 0.011 a) — — — — — 14 0.0011 0.009 0.005 0.008 a) — — — — — 15 0.0009 0.018 0.011 0.015 a) — — — — — 16 0.0009 0.015 0.004 — — — — — — 17 0.0012 0.014 0.004 0.014 a) — — — — — An underlined numerical value is outside the range defined by the invention.

The numerical values in Table 2 refer to the contents of corresponding elements (% by mass). Among these chemical compositions, the balance other than the elements described in Table 1 includes Fe and impurities. The symbols a) to c) attached to the numerical values in the “REM” column each represent the kind of REM included in the steel. More specifically, a) means that the contained REM is neodymium (Nd). Similarly, b) means that the contained REM is yttrium (Y) and c) means that the contained REM is misch metal. The misch metal contains, in percent by mass, 51.0% cerium (Ce), 25.5% lanthanum (La), 18.6% neodymium (Nd), 4.8% praseodymium (Pr) and 0.1% samarium (Sm).

With reference to Table 2, the steel kinds with Nos. 1 to 12 each had a chemical composition within the range defined by the invention. Regarding the steel with No. 13, its Mo content was less than the lower limit defined by the invention. Regarding the steel with No. 14, its Cr content was less than the lower limit defined by the invention. Regarding the steel with No. 15, its Cu content was less than the lower limit defined by the invention. The steel with No. 16 contained no REM. Regarding the steel with No. 17, its Ni content was less than the lower limit defined by the invention.

These numbered steels were each subjected to hot forging and hot rolling, and a steel plate having a thickness of 12 mm was produced. The numbered steel plates were subjected to quenching and tempering. In the quenching processing, the steel plates were each heated for 15 minutes at a quenching temperature from 980° C. to 1200° C. and then cooled with water. In the tempering processing, the tempering temperature was from 500° C. to 650° C. Through these steps, the yield stress of each steel plate was adjusted to be in the range from 800 MPa to 950 MPa.

Structure Observation and Tensile Tests

The volume percentage (%) of the ferrite phase and the residual austenite phase of each steel plate was obtained by the measuring method described in 3.

Then, a round rod tensile test specimen was sampled from each of the steel plates and subjected to a tensile test. The length-wise direction of the round rod tensile test specimen was arranged in the direction of rolling the steel plate and the parallel part of the round rod tensile test specimen had a diameter of 14 mm, and a length of 20 mm. The tensile tests were carried out at room temperatures.

SCC Evaluation Tests

A four-point bending specimen having a length of 75 mm, a width of 10 mm, and a thickness of 2 mm was sampled from each of the steel plates. Each of the sampled specimens was bent by four-point bending. At the time, according to ASTM G39, the amount of bending of each of the specimens was determined so that stress applied on each of the specimens was equal to the yield stress of each of the specimens.

The bent specimens were each immersed for one month in a 25 wt % NaCl aqueous solution in an autoclave at 204° C. (400 F) having CO₂ enclosed therein under a pressure of 30 atm. After the immersion for one month, the specimens were examined for the presence of SCC. More specifically, a longitudinal section of each specimen was observed with a 100× magnification optical microscope and examined for the presence/absence of SCC by visual inspection. The weight of each specimen was measured before and after the test. From the difference between the measured weights, the weight loss of each specimen caused by corrosion was obtained and the corrosion rate was calculated based on the weight loss.

Test Result

The test result is given in Table 3.

TABLE 3 Ferrite Austenite phase in phase in volume volume SCC Corrosion Steel YS percentage percentage evaluation rate No. (MPa) (%) (%) result (g/(m² · hr)) 1 924 25 2.1 NO SCC <0.1 2 915 26 5.2 NO SCC <0.1 3 901 28 5.6 NO SCC <0.1 4 893 35 3.2 NO SCC <0.1 5 940 15 4.2 NO SCC <0.1 6 886 38 2.2 NO SCC <0.1 7 922 20 4.1 NO SCC <0.1 8 928 21 4.5 NO SCC <0.1 9 926 24 3.6 NO SCC <0.1 10 933 19 6.8 NO SCC <0.1 11 911 28 5.2 NO SCC <0.1 12 903 33 3.3 NO SCC <0.1 13 880 38 2.3 SCC <0.1 PRESENT 14 932 20 4.9 SCC ≧0.1 PRESENT 15 873 42 2.0 SCC <0.1 PRESENT 16 899 30 3.5 SCC <0.1 PRESENT 17 880 35 2.8 SCC <0.1 PRESENT

The “YS” column in Table 3 represents the yield stress (MPa) of each of the numbered steel plates obtained by the tensile tests. The “ferrite phase” and “residual austenite phase” columns represent the volume percentages (%) of the ferrite phase and the residual austenite phase in each of the steel plates. In the “SCC evaluation result” column, the “NO SCC” indicates that there was no SCC generated at the four-point bending test specimen, and the “SCC PRESENT” indicates that there was SCC. In the “corrosion rate” column, the “<0.1” indicates that the corrosion rate was less than 0.1 g/(m²·hr), while the “≧0.1” indicates that the corrosion rate was not less than 0.1 g/(m²·hr).

With reference to Table 3, the steels with Nos. 1 to 12 did not have any SCC and their corrosion rates were all less than 0.1 g/(m²·hr). Their yield stress values were all 654 MPa or more.

On the other hand, the steels with Nos. 13, 15 and 17 had SCC because their Mo, Cu, and Ni contents were small. The steel with No. 14 had SCC because it contained only a small amount of Cr, and its corrosion rate was not less than 0.1 g/(m²·hr). Furthermore, the steel with No. 16 had SCC because it did not contain REM.

While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.

Applicable Field in the Industry

The stainless steel according to the invention can be applied as oil country tubular goods and particularly suitably applied to an oil country tubular good for use in a carbon dioxide gas contained, high temperature chloride aqueous solution environment at 150° C. or higher. 

1. Stainless steel used for oil country tubular goods, comprising, in percent by mass, 0.001% to 0.05% C, 0.05% to 1% Si, at most 2% Mn, at most 0.03% P, less than 0.002% S, 16% to 18% Cr, 3.5% to 7% Ni, more than 2% and at most 4% Mo, 1.5% to 4% Cu, 0.001% to 0.3% rare earth metal, 0.001% to 0.1% sol. Al, 0.0001% to 0.01% Ca, at most 0.05% O, and at most 0.05% N, the balance consisting of Fe and impurities.
 2. The stainless steel according to claim 1, further comprising, in place of a part of Fe, at least one selected from the group consisting of at most 0.5% Ti, at most 0.5% Zr, at most 0.5% Hf, at most 0.5% V, and at most 0.5% Nb.
 3. The stainless steel according to claim 1, having a structure including, in percent by volume, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.
 4. The stainless steel according to claim 2, having a structure including, in percent by volume, 10% to 60% ferrite phase and 2% to 10% residual austenite phase.
 5. The stainless steel according to claim 1, having a yield stress of at least 654 MPa.
 6. The stainless steel according to claim 2, having a yield stress of at least 654 MPa.
 7. The stainless steel according to claim 3, having a yield stress of at least 654 MPa.
 8. The stainless steel according to claim 4, having a yield stress of at least 654 MPa. 